Thermal Reactor Comprising a Gas Permeable Cage Arranged to Influence a Flow Path of Gas

ABSTRACT

There is provided a method for the synthesis of nitrogen oxides (NOx) comprising the steps of providing a gas mixture comprising oxygen and nitrogen; and heating the gas mixture to a temperature of at least 2300 K at a pressure of 10-100 bar in a thermal reactor forming a gas mixture comprising NOx. There is also provided a method for the production of HNO3.

TECHNICAL FIELD

The present disclosure relates to the production of nitrogen oxides (NO_(x)) and in particular to production of NO, NO₂ and HNO₃.

BACKGROUND

Nitrogen oxides (NO_(x)) are regularly used in production of nitric acid, which is used for the production of ammonium nitrate, a major component of fertilizera, for the production of explosives, as solvents and for other chemical processes, as well as for bleaching and sterilization. Nitrogen is a key element for plants and therefore one of the most important nutrients in fertilizers. In the last century, nitrogen-containing fertilizers have been produced from atmospheric nitrogen essentially through the Haber-Bosch process to produce ammonia from hydrogen originating from fossil-based steam reforming and molecular nitrogen followed by the Ostwald process to produce nitric acid through oxidation of the ammonia. The nitric acid is then used for example as a source for producing nitrate fertilizers.

Alternatively, nitric acid can be produced through nitrogen fixation through the Birkeland-Eyde process, invented in 1903. By the aid of an electric arc, a thermal oxidation of atmospheric nitrogen into NO is conducted, and the NO spontaneously converts to NO₂ as the gas is cooled in the presence of oxygen. NO₂ is subsequently scrubbed with water and, thereby, converted into nitric acid. However, the thermal oxidation of nitrogen gas is highly energy demanding and therefore the Haber-Bosch and Ostwald processes became the leading processes used commercially to fix nitrogen from the air and to produce nitric acid.

Due to the emissions of greenhouse gases (CO₂, NO, N₂O) from the steam reforming and Haber-Bosch process combined with the Ostwald process, thermal oxidation of nitrogen has returned as an interesting option. However, due to the high energy demands of the Birkeland-Eyde process, there are improvements to be made especially regarding energy efficiency.

SUMMARY

One objective is to make available an essentially carbon neutral and industrially feasible method being a more energy efficient route compared to previous thermal oxidation methods for producing nitrogen oxides (NO_(x)) as well as nitric acid (HNO₃).

There is thus, as a first aspect of the present disclosure, provided a method for the synthesis of nitrogen oxides (NO_(x)) comprising the following steps:

-   -   providing a gas mixture comprising oxygen and nitrogen; and     -   heating the gas mixture to a temperature of at least 2300 K at a         pressure of 10-100 bar in a thermal reactor forming a gas         mixture comprising NO_(x).

There is also provided, as a second aspect of the present disclosure, a method for the synthesis of nitric acid (HNO₃) comprising the following steps:

-   -   providing a gas mixture comprising oxygen and nitrogen;     -   heating the gas mixture to a temperature of at least 2300 K at a         pressure of 10-100 bar in a thermal reactor forming a gas         mixture comprising NO_(x);     -   cooling the gas mixture in a cooling step forming a cooled gas         mixture comprising NO₂, wherein the cooling step comprises         quenching the gas mixture comprising NO_(x);     -   optionally, depressurizing the cooled gas mixture comprising         NO₂;     -   wet scrubbing the gas mixture comprising NO₂ forming HNO₃; and     -   optionally, recirculating unreacted exhaust gas from the wet         scrubbing to the thermal reactor.

The combination of temperature and pressure decreases the energy consumption in the formation of NO_(x) from oxygen and nitrogen gas. The inventors have found that by applying a pressure of 10-100 bar and a temperature above 2300 K there is a pronounced effect of reduction of the energy needed for formation of NO_(x).

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The method steps disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic image of a process for the synthesis of nitrogen oxides (NO_(x)).

FIG. 2 shows a schematic image of a process for the synthesis of HNO₃, including an oxygen sieve/oxygen concentrator, a turbine and a scrubber.

FIG. 3 shows the results from a simulation of the formation of NO from N₂ and O₂ at increasing temperature and pressure.

FIG. 4 shows the results from a simulation of the formation of NO from N₂ and O₂ at 0.05 bar and 50 bar.

Detailed descriptions of the drawings are presented below.

DETAILED DESCRIPTION

As a first aspect of the present disclosure, there is provided a method for the synthesis of nitrogen oxides (NO_(x)) comprising the following steps:

-   -   providing a gas mixture comprising oxygen and nitrogen; and     -   heating the gas mixture to a temperature of at least 2300 K at a         pressure of 10-100 bar in a thermal reactor forming a gas         mixture comprising NO_(x).

In the gas mixture comprising oxygen and nitrogen, the oxygen is typically oxygen gas (O₂) and the nitrogen is typically nitrogen gas (N₂).

By nitrogen oxides (NO_(x)) is meant compounds produced by reacting nitrogen and oxygen. Typically, the nitrogen oxides are nitric oxide (NO) and/or nitrogen dioxide (NO₂). Principally, the NO_(x) formed by heating the gas mixture comprising oxygen and nitrogen to a temperature of at least 3000 K at a pressure of 10-100 bar in a thermal reactor is NO.

The thermodynamic balance in formation of NO₂ from N₂ and O₂ is:

N₂+O₂

2NO  (1)

2NO+O₂

NO₂  (2)

The energy consumption for the formation of the NO_(x), NO, from nitrogen and oxygen is significantly reduced upon application of a pressure above 10 bar. This is shown in FIGS. 3-4 , and further laid out in the example.

The yield, i.e. the equilibrium concentration of NO in reaction step (1), in the formation of NO is typically up to 11%, such as 4-10%.

The pressure of the gas mixture comprising oxygen and nitrogen is 10-100 bar, preferably 15-70 bar, more preferably 15-60 bar, even more preferably 15-50 bar. In some embodiments, the pressure may be above 20 bar, such as 23-100 bar, such as 23-70 bar. The increase in pressure is typically obtained by compressing the gas mixture in a compressor.

The temperature of the gas mixture comprising oxygen and nitrogen is at least 2300 K, preferably at least 2500 K, more preferably at least 2800 K, even more preferably at least 3000 K. Moreover, the temperature is preferably at most 4500 K, more preferably at most 4000 K, wherein the upper limits are freely combinable with the lower limits, such that the temperature of the gas mixture comprising oxygen and nitrogen is preferably 2300 K-4500 K, more preferably 2300 K-4000 K, even more preferably, 2500 K-4500 K, even more preferably, 2500 K-4000 K, even more preferably 2800 K-4500 K, even more preferably, 2800 K-4000 K, even more preferably 3000 K-4500 K, even more preferably, 3000 K-4000 K.

The thermal reactor is typically constructed in a material that is both temperature resistant and mechanically resistant, capable of withstanding both the load from the pressure difference and the temperature. Examples of such material include MgO or the composite HfB2/20% SiC that has a high oxidation resistance and can resist temperatures above 2100 K. Further examples can be found in the article “Super-strong materials for temperatures exceeding 2000° C., L. Silvestroni et al., Sci Rep. 2017; 7”.

Alternatively, the reactor wall can be cooled to a temperature where it has enough strength to carry the pressure difference. Special flow schemes can also be used so that the contact between the hot gas from the plasma with the reactor wall can be avoided or minimized. The “reversed vortex flow” is one example of such gas flow schemes.

Examples of reactor set-ups that can be used include the setup described by Hans-Peter Schmidt and Günter Speckhofer (Schmidt, H -P., and G. Speckhofer. “Experimental and theoretical investigation of high-pressure arcs. L The cylindrical arc column (two-dimensional modeling).” IEEE Transactions on plasma science 24.4 (1996): 1229-1238.), where a gas plasma reactor reaching argon temperatures above 20 000 K at a pressure of 100 bar was created in a bell-shaped Pyrex® glass. Another example of a reactor is available from HiiROC Ltd, UK, capable of producing a plasma at a pressure of 50 bar.

The thermal reactor is preferably a plasma reactor. By plasma reactor is meant a reactor where the temperature applied in the reactor contributes to reaching conditions where the gas can be ionized and, at least partially, form a plasma.

The thermal reactor is preferably operated by heating the gas mixture comprising oxygen and nitrogen with radiofrequency waves or microwaves. The radiofrequency waves or microwaves transfer energy to the gas molecules. In such a case, a radiofrequency wave plasma or a microwave plasma, can be formed in a thermal reaction zone. If a plasma is formed, the radiofrequency wave or microwave energy input couples primarily with the dissociated electrons in the plasma. The radiofrequency waves or microwaves also transfer energy to the molecules. The use of radio waves or microwaves is beneficial since it is possible to focus the energy to a sector, in principle an arbitrary sector, of the reactor without requiring that any electrodes are in direct contact with the heated thermal reaction zone. If the electrodes are required to be in the vicinity of the thermal reaction zone, or even in direct contact, the electrodes may become so hot that they are consumed. One alternative solution is to limit the temperature, but that will reduce the efficiency of the process.

The gas mixture comprising oxygen and nitrogen typically has an oxygen content of 25-60% (vol/vol), preferably 25-55% (vol/vol). Such oxygen content decreases the energy input needed due to a more balanced formation of NO_(x). Such oxygen content is typically provided by running a gas mixture comprising oxygen and nitrogen, such as air, through an oxygen sieve or oxygen enriching device. Molecular sieves for separating/enriching gases are well-known to persons skied in the art, and available from commercial suppliers. Alternatively, the gas mixture comprising oxygen and nitrogen has an oxygen content of 20-25% (vol/vol) that is advantageous because it avoids the need for enriching the gas mixture with oxygen.

The gas mixture comprising the formed NO_(x) is typically cooled in a cooling step forming a cooled gas mixture comprising NO₂, wherein the cooling step comprises quenching the gas mixture comprising NO_(x).

The quenching is typically conducted until obtaining a temperature sufficiently low so that the NO_(x) in the cooled gas mixture principally is NO and the NO essentially does not reverse into N₂ and O₂ according to the equilibrium (1). The quenching is typically conducted directly downstream from the thermal reaction in the thermal reactor. Typically, the gas mixture comprising NO_(x) is quenched by lowering the temperature below 2200 K, preferably below 2000 K, more preferably below 1900 K, even more preferably below 1700 K.

In one embodiment, the gas mixture is quenched to a temperature below 750 K, preferably to a temperature below 373 K, more preferably to a temperature between 293-363 K, wherein the formation of NO₂ in accordance with equilibrium (2) is promoted.

In an alternative embodiment, the gas mixture is quenched followed by a second cooling step, wherein in said second cooling step the gas mixture is cooled to a temperature below 750 K, preferably to a temperature below 373 K, more preferably to a temperature between 293-363 K. The second cooling step typically cools at a cooling rate being at least 10% lower than the cooling rate of the quenching. In such an embodiment, the formation of NO₂ in accordance with equilibrium (2) is promoted during the second cooling step. The lower cooling rate of the second cooling step is beneficial since energy recovery is facilitated.

Preferably, the quenching is conducted by bringing the gas mixture in contact with a quenching medium. The quenching medium is preferably water or a gas mixture comprising oxygen and nitrogen. Typically, the gas mixture comprises oxygen, nitrogen and NO recirculated from the cooling step and/or oxygen and nitrogen that was deliberately by-passed the thermal reactor. In the case where the gas mixture comprises oxygen, nitrogen and NO recirculated from the cooling step, the gas is preferably recirculated back after conducting the quenching to be mixed with gas from the thermal reactor. This is beneficial since it is energy efficient. Alternatively, the gas is recirculated back after the complete cooling step. Prior to being used as quenching medium, i.e. mixed with gas from the reactor, the gas mixture is typically cooled in a heat-exchanger capable of recovering energy for example by the generation of steam to be used in a steam turbine e.g. to produce electricity. The cooled gas mixture is thereafter injected into the quenching step. Such gaseous quenching medium is beneficial since the energy efficiency is higher compared with water as quenching medium since no energy is needed to vaporize water. Moreover, if a turbine is used, the risk of water in the turbine is reduced.

Water as quenching medium is beneficial particularly when no turbine is used. In such a case, water is added to the gas through sprinklers with the purpose of conserving the NO equilibrium content in the gas mixture comprising NO), according to equilibrium (1).

In the cooling step, the gas mixture is typically depressurized, preferably during the second cooling step subsequent to quenching. Alternatively, the pressure is maintained throughout the cooling step. In such a case, HNO₃ can be produced at high pressure, extracted from the process to atmospheric pressure while the exhaust gas can be recirculated to the reactor.

While the temperature is lowered in the cooling step, energy can be extracted from the gas mixture. At least part of the extraction of energy is conducted in a turbine system, wherein the turbine system preferably comprises a cooler or condenser. A system comprising a compressor pumping in the gas mixture comprising oxygen and nitrogen to maintain the pressure, and a turbine placed downstream after the thermal reactor is beneficial. It has been estimated that by using a compressor and turbine in combination, it is possible to recover approx. 40-45% of the energy put into the system, however depending on for example the scale of application.

According to an alternative embodiment, the cooling step is performed by leading the gas mixture comprising NO), into a heat-exchanger of suitable dimensions. The heat-exchanger can be operated to produce steam either directly, or via a secondary circulation. The produced steam can be used for heating purposes, or for the generation of electricity in a steam turbine.

The gas mixture comprising NO₂ is wet scrubbed forming HNO₃. The wet scrubbing is preferably conducted after the cooling step.

In one embodiment, the cooling step is conducted and followed downstream by wet scrubbing without depressurizing with for example a turbine. This can facilitate recirculation of unreacted exhaust gas to the reactor, as the pressure will not drop significantly, and less energy is needed to re-pressurize the gas to the desired pressure for the thermal reactor. Moreover, HNO₃ can be produced at high pressure, and typically it is only the HNO₃ extracted from the process that is depressurized to atmospheric pressure. In addition, since liquids are in general incompressible, the loss of work is kept low.

Typically, unreacted exhaust gas is recirculated from the scrubber to the thermal reactor and may be conducted both if the gas mixture is maintained at high pressure or has been depressurized. Recirculation is beneficial since the recirculated gas mixture already has an elevated oxygen content so only the amount of N₂ and O₂ that has been reacted has to be introduced from the exterior. In the case where the gas mixture is maintained at high pressure a further advantage is that due to the lower amount of new gas provided and the gas already being pressurized, the demand for large compressors is reduced. Similarly, the need for a large turbine can be reduced since the gas does not have to be depressurized. In the case where the gas has been depressurized, the gas is recirculated after depressurizing in for example a turbine. In such a case, a compressor is applied, but the capacity of the oxygen sieve/oxygen enriching device can be reduced compared to without recirculation. It is estimated that approx. 10% of the capacity is needed.

As a second aspect of the present disclosure, there is provided a method for the synthesis of nitric acid (HNO₃) comprising the following steps:

-   -   providing a gas mixture comprising oxygen and nitrogen;     -   heating the gas mixture to a temperature of at least 2300 K at a         pressure of 10-100 bar in a thermal reactor forming a gas         mixture comprising NO_(x);     -   cooling the gas mixture in a cooling step forming a cooled gas         mixture comprising NO₂, wherein the cooling step comprises         quenching the gas mixture comprising NO_(x);     -   optionally, depressurizing the cooled gas mixture comprising         NO₂;     -   wet scrubbing the gas mixture comprising NO₂ thereby forming         HNO₃; and     -   optionally, recirculating unreacted exhaust gas from the wet         scrubbing to the thermal reactor.

What is described above with respect to the first aspect applies to the second aspect mutatis mutandis.

The aspects of the present disclosure will now be described hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown.

These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like numbers refer to like elements throughout the description.

FIG. 1 shows a schematic image of a process for the synthesis of nitrogen oxides comprising providing a gas mixture comprising oxygen (O₂) and nitrogen (N₂); compressing the gas mixture in a compressor 101 into a pressure P and heating the compressed gas mixture in a thermal reactor 102 to form a gas mixture comprising NO_(x).

FIG. 2 shows a schematic image of a process for the synthesis of nitric acid (HNO₃). The process comprises the steps of:

-   -   providing a gas mixture comprising oxygen (O₂) and nitrogen         (N₂);     -   increasing the oxygen content in the gas mixture in an oxygen         sieve 100;     -   compressing the gas mixture in a compressor 101 into a pressure         P;     -   heating the compressed gas mixture in a thermal reactor 102 to         form a gas mixture comprising NO_(x);     -   cooling the gas mixture by quenching in a quencher 103;     -   depressurizing the gas in a turbine with a generator 104 to form         depressurized NO₂ while recovering energy 106 to the compressor         and/or thermal reactor; and     -   wet scrubbing the NO₂ in a scrubber 105 thereby forming HNO₃.

EXAMPLE

Simulation of the Formation of NO from N₂ and O₂ at Increasing Temperature and Pressure

The simulation was performed using MATLAB HGS Chemical Equilibrium Calculation code, which is an implementation of NASA Computer program CEA. It minimizes the Gibbs free energy for the specified temperature, pressure, and reactants, to arrive at an equilibrium composition. Details are found in: S. Gordon, B. J. McBride, NASA Reference Publication 1311, 1994; and in B. J. McBride, S. Gordon, NASA Reference Publication 1311, 1996.

The simulations were conducted for a stochiometric gas mixture of N₂ and O₂, i.e. 50% N₂ and 50% O₂ (vol/vol), as well as a gas mixture similar to that of air having 80% N₂ and 20% O₂ (vol/vol). Calculations were made for every 100 K increase in temperature.

The results are presented in FIG. 3 . The lines representing the stochiometric gas mixture are dashed and the lines representing the gas mixture similar to air are unbroken. The fine lines (the lines going upwards as the pressure increases) show the temperature at which the energy consumption is the lowest per kg NO, while the bold lines (the lines going downwards) show the optimized energy consumption (kJ/mol) for the formation of NO at a given temperature (shown by the line of regular thickness going upwards) and pressure.

Nitrogen oxide formation in a plasma is limited by the thermodynamic equilibrium concentration that is approached in the thermal core of the plasma. The graph in FIG. 3 depicts how the thermodynamic equilibrium limitation improves towards pressure. The plasma reactor performance will qualitatively follow this trend with pressure.

In Table 1 the data for the optimized energy consumption for both gas mixtures are presented at three different pressures: 1, 10 and 50 bar.

TABLE 1 Optimized energy consumptions for the formation of NO from N₂ and O₂. Gas mixture Pressure Optimized energy consumption Temperature (vol/vol ratio) (bar) at a certain pressure (kJ/mol) (K) 80:20 - N₂:O₂ 1 2600 3100 80:20 - N₂:O₂ 10 2050 3500 80:20 - N₂:O₂ 50 1800 3750 50:50 - N₂:O₂ 1 2250 3000 50:50 - N₂:O₂ 10 1750 3500 50:50 - N₂:O₂ 50 1500 3750

The energy consumption decreases by approx. 20% when increasing the pressure from 1 bar to 10 bar, and an additional approx. 10% when the pressure is increased to 50 bar. At a pressure above 100 bar the effect is less pronounced, and the energy consumption curves level off.

FIG. 4 illustrates further results from the simulation being the energy cost, i.e. energy consumption, per NO formed from O₂ and N₂ as a function of temperature at two different pressures: 0.05 bar and 50 bar, for the two gas mixtures. The lines representing the stochiometric gas mixture are dashed and the lines representing the gas mixture similar to air are unbroken. The lines of regular thickness are displaying the energy consumption at 0.05 Bar, while the bold lines are displaying the energy consumption at 50 Bar. As displayed in FIG. 4 , the formation of NO also occurs at lower temperatures than the ones presented in FIG. 3 , but with a higher energy cost per NO formed, which results in a lower yield.

In Table 2 the data for the energy consumption for the formation of NO at 2300 K from both gas mixtures are presented at the two different pressures: 0.05 bar and 50 bar.

TABLE 2 Energy consumptions for the formation of NO from N₂ and O₂ at 0.05 bar and 50 bar. Gas mixture Pressure Energy consumption for Temperature (vol/vol ratio) (bar) formation of NO (kJ/mol) (K) 80:20 - N₂:O₂ 0.05 4750 2300 80:20 - N₂:O₂ 50 4500 50:50 - N₂:O₂ 0.05 3900 50:50 - N₂:O₂ 50 3600 80:20 - N₂:O₂ 0.05 4000 2500 80:20 - N₂:O₂ 50 3550 50:50 - N₂:O₂ 0.05 3350 50:50 - N₂:O₂ 50 2900

The energy consumption decreases with approx. 10% when increasing the pressure from 0.05 bar to 50 bar at both 2300 K and 2500 K for the stochiometric gas mixture. For the gas mixture similar to air the energy consumption decreases with approx. 5% at 2300 K and approx. 10% at 2500 K. There is, thus, a pronounced reduction in energy efficiency at temperatures below the temperature wherein the energy consumption is optimized.

The aspects of the present disclosure have mainly been described above with reference to a few embodiments and examples thereof. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

1. A thermal reactor (100) comprising: a vessel (101), said vessel comprising: a gas inlet (102), a gas permeable cage (104) arranged in the vessel (101), and in fluid connection to the gas inlet (102), wherein the vessel (101) and the cage (104) are provided with a mutual gas outlet (103), and temperature generating means (105;105′) arranged to create a thermal reaction zone (106) within the cage (104), wherein the cage (104) is provided with holes (107), and wherein a first subset of the holes (107′) is arranged along at least a portion of a first circumferential surface (110) of the cage (104) and a second subset of the holes (107″) is arranged along at least a portion of a second circumferential surface (111) of the cage (104), wherein the first (110) and second (111) circumferential surfaces are offset and non-parallel, and the first subset of holes (107′) and the second subset of holes (107″) are mutually distinct.
 2. The thermal reactor according to claim 1, wherein the thermal reactor (100) is a plasma reactor (100), the thermal reaction zone is a plasma zone (106) and the temperature generating means (105;105′) are plasma generating means (105;105′).
 3. The thermal reactor according to claims 1, wherein the vessel (101) is a pressurized vessel arranged to operate at different pressure than atmospheric pressure.
 4. The thermal reactor according to claim 1, wherein the vessel (101) further comprises cooling means (108).
 5. The thermal reactor according to claim 4, wherein the cooling means (108) are arranged in the outlet (103) or in direct connection to the outlet (103).
 6. The thermal reactor according to claim 1, wherein the cage (104) is porous.
 7. The thermal reactor according claim 1, wherein the cage (104) is a metal cage.
 8. The thermal reactor according to claim 1, wherein the cage (104) is a ceramic cage.
 9. The thermal reactor according to claim 1, wherein the cage (104) is made of a non-metallic conductive material.
 10. The thermal reactor according to claim 1, wherein the temperature generating means (105; 105′) are electrodes.
 11. The thermal reactor according to claim 1, wherein the temperature generating means (105; 105′) are antennas.
 12. The thermal reactor according to claim 1, wherein the thermal reaction zone (106) is produced using electromagnetic waves of radio frequency or microwaves.
 13. The thermal reactor according to claim 1, wherein at least 80% of the holes (107), such as at least 90% of the holes (107), have a central axis (Y) that is angled at an angle a being between 80°-100° relative to a tangential plane (X) at an outer surface of the cage around respective hole (107).
 14. The thermal reactor according to claim 1, wherein the cage (104) has rounded edges.
 15. The thermal reactor according to claim 1, wherein the cage (104) is an ellipsoid.
 16. The thermal reactor according to claim 1, wherein the cage (104) is a cylinder.
 17. The thermal reactor according to claim 1, wherein the cage (104) has a central longitudinal axis around which the cage (104) is symmetrical.
 18. The thermal reactor according to claim 1, wherein the surface geometry of the cage (104) can be described by a continuous function.
 19. The thermal reactor according to claim 18, wherein the derivative of the continuous function describing the surface geometry of the cage (104) is a continuous function.
 20. The thermal reactor according to claim 19, wherein the second derivative of the continuous function describing the surface geometry of the cage (104) is a continuous function.
 21. The thermal reactor according to claim 1, wherein at least 30%, such as at least 40%, such as at least 50%, such as at least 60%, such as at least 70%, such as at least 80%, such as at least 90%, of the surface of the cage (104) is provided with holes (107).
 22. The thermal reactor according to claim 21, wherein the entire surface of the cage (104) is provided with holes (107).
 23. The thermal reactor according to claim 1, wherein the holes (107) are substantially circular.
 24. The thermal reactor according to claim 1, wherein the cage (104) is spaced from the walls of the vessel (101).
 25. The thermal reactor according to claim 1, wherein the gas permeable cage (104) is a first gas permeable cage (104-1) and the thermal reactor further comprises: a second gas permeable cage (104-2), wherein the holes (107) of the first gas permeable cage (104-1) are first holes (107-1), and the second gas permeable cage (104-2) is provided second holes (107-2), wherein the second gas permeable cage (104-2) is smaller than the first gas permeable cage (104-1), so that the second gas permeable cage (104-2) is arranged inside the first gas permeable cage (104-1).
 26. The thermal reactor according to claim 25, wherein the first and second holes (107-1, 107-2) of the first and second gas permeable cages (104-1, 104-2) are arranged offset so that the first and second holes (107-1, 107-2) are not aligned.
 27. The thermal reactor according to claim 25, wherein the first gas permeable cage (104-1) and the second gas permeable cage (104-2) have the same geometrical shape. 